(a) Field of the Invention
The present invention relates to a multi DFB (distributed feedback) laser diode. More specifically, the present invention relates to a multi DFB laser diode improved to have a smooth self-pulsing operation.
(b) Description of the Related Art
In an optical communication system, clock recovery for 3R regeneration (re-timing, re-shaping, and re-amplifying) is carried out through an optical pulse device. Conventionally, an optical pulse is generated using Q-switching or phase locking that oscillates beams of more than two modes whose relative phases are locked, and makes them interfere with each other. Q-switching is not suitable for high-speed optical communications because its speed is limited by modulation bandwidth, but phase locking can be utilized without having restriction on the speed since a pulse repetition rate is determined by the interval between modes. In particular, a mode-locked laser diode and a multi DFB laser diode employing phase locking are widely used for optical communication systems.
In case of the mode-locked diode, injection locking is difficult to perform at a bit rate of more than 40 GHz, and its power is very low at several mW. Accordingly, studies on the multi DFB laser diode are being undertaken, but it also has problems. First of all, when the period of a diffraction grating is not in accord with two DFB sections accurately, a light intensity difference become asymmetrical so that the modulation index of the pulse generated is deteriorated. To solve this problem, the period of the diffraction grating is controlled through e-beam lithography. However, this method is costly and not suitable for mass production. Furthermore, generation of a single mode, which is a fundamental problem of the DFB laser diode, is problematical in terms of production yield.